The present invention relates generally to methods and apparatus for phase shifting at least one of a plurality of optical signals to obtain a predetermined phase relationship between the optical signals and, more particularly, to a method and apparatus for controllably positioning an optical fiber in order to adjust the phase of the optical signals emitted by the respective optical fiber relative to the optical signals emitted by other optical fibers such that the predetermined phase relationship is obtained.
It is desirable in many applications to generate laser radiation having relatively high power levels. For example, a variety of military, material processing, medical, and communications applications demand laser radiation having relatively high power levels. One technique for generating these relatively high power levels is to combine or sum the optical signals emitted by a plurality of laser sources, all of which typically derive their origin from a common source. As described in U.S. Pat. No. 5,694,408 to Eric H. Bott, et al., the contents of which are incorporated herein by reference, the laser signals emitted by a laser signal source can be split into a plurality of secondary laser signals, each of which can be amplified, such as by means of a fiber amplifier or other laser amplifier. The amplified secondary laser signals can then be recombined in order to generate an output beam capable of having relatively high power levels. In order to properly combine a plurality of optical signals to form a single diffraction limited spot in the far field, however, the optical signals should be coherent.
Optical signals combine by vector summation of their electromagnetic fields. Combined coherent optical signals may sum or cancel depending on the relative optical phase of the combined electromagnetic fields. Relative phases of the combined signals vary with motion or displacement of the optical fibers that constitute the paths of the different signals. Even an acoustic wave passing through the fiber environment can noticeably affect the differential optical phase between the plurality of paths.
In order to ensure that the coherent optical signals are in the proper phase for optimal combination, the phase of the optical signals must be adjusted such that each of the optical signals is matched in phase (phase matched) with the other optical signals. A variety of techniques have been developed for phase shifting the optical signals emitted by an optical fiber. For example, U.S. Pat. Nos. 4,703,287 and 4,814,774 describe piezoelectric elements that stretch the medial portion of an optical fiber in order to alter the phase of the signals emitted by the optical fiber. In this regard, stretching of an optical fiber introduces a time delay to a path length change which is equivalent to a change in phase of the signals emitted by the optical fiber. Alternatively, current control or reverse bias semiconductor junction voltage controlled phase shifters have been developed for introducing phase shifts. Moreover, electro-optic modulators, such as those described by U.S. Pat. No. 5,694,408, have been developed in order to controllably adjust the phase of the optical signals emitted by each of a plurality of optical fibers.
While these prior techniques for adjusting the phase of the optical signals emitted by each of a plurality of optical fibers can be effective for phase matching which, in turn, is necessary to permit the coherent combination of the plurality of optical signals, these prior techniques are oftentimes quite expensive and trouble prone. As such, it would be desirable to provide an improved technique for adjusting the phase of the optical signals emitted by each of a plurality of optical fibers in order to facilitate the coherent combination of the plurality of optical signals in a less expensive and more reliable fashion.
An apparatus and method are therefore provided for controllably phase shifting at least one of a plurality of optical signals by controllably positioning the end portion of the optical fiber that emits the optical signals to be phase shifted relative to the end portions of a number of other optical fibers that emit the remainder of the optical signals. The optical path length change introduces a change in propagation time delay which is equivalent to a change in phase of the signals emitted by the optical fiber. By utilizing a micromanipulator to controllably position the end portion of the respective optical fiber, the method and apparatus of the present invention can reliably and relatively inexpensively control the phases of a plurality of optical signals in order to maintain a predetermined phase relationship, such as by phase matching each of the plurality of optical signals.
The phase shifting apparatus includes a plurality of optical fibers for supporting the propagation of optical signals from a first end to an opposed second end from which the optical signals are emitted. In order for the optical signals transmitted via each of the optical fibers to have the same frequency, the apparatus typically includes a common optical source for providing optical signals to each of the optical fibers. The phase shifting apparatus also includes at least one micromanipulator for controllably positioning the second end of a respective optical fiber relative to the second ends of the other optical fibers, thereby shifting the phase of the optical signals emitted by the respective optical fiber. In this regard, the phase shifting apparatus can include a controller for directing the micromanipulator in order to controllably position the second end of the respective optical fiber. By controllably positioning the second end(s) of one or more of the optical fibers, the phase shifting apparatus can establish a predetermined phase relationship between the optical signals emitted by each of the optical fibers.
In one advantageous embodiment, the phase shifting apparatus includes a plurality of micromanipulators, one of which is associated with each optical fiber. As such, the micromanipulators can controllably position the second ends of the respective optical fibers such that the optical signals emitted by the plurality of optical fibers are phase matched. Typically, the optical signals have a predetermined direction of propagation following emission by an optical fiber. As such, each micromanipulator preferably moves the second end of the respective optical fiber along, or parallel to, the predetermined direction of propagation in order to shift the phase of the optical signals. Although the phase shifting apparatus can include a variety of micromanipulators, the phase shifting apparatus of one advantageous embodiment includes a micromanipulator having a carrier upon which the second end of the respective optical fiber is mounted and at least one actuator element for moving both the carrier and the second end of the respective optical fiber relative to the other optical fibers.
In operation, optical signals having a predetermined wavelength are provided to a plurality of optical fibers. Following transmission of the optical signals along the plurality of optical fibers from a first end to an opposed second end, the optical signals are emitted. In order to establish a predetermined phase relationship between the optical signals emitted by the optical fibers, the second end of at least one optical fiber is controllably positioned relative to the second ends of the other optical fibers in order to shift the phase of the optical signals emitted by the respective optical fiber. For example, the second end of a respective optical fiber can be controllably positioned by moving the second end of the respective optical fiber in the predetermined direction in which the optical signals propagate following emission by the optical fibers. Typically, the second end of each optical fiber is controllably positioned such that the optical signals emitted by each of the optical fibers are phase matched.
By phase shifting the optical signals emitted by one or more of the optical fibers, the method and apparatus of the present invention can match the phase of the optical signals. As such, the optical signals can then be coherently combined in order to generate an output beam having much higher power than any of the individual optical signals. Moreover, by phase shifting the optical signals emitted by one or more of the optical fibers by controllably positioning the end portions of the optical fibers with a micromanipulator or the like, the method and apparatus of the present invention can establish the predetermined phase relationship between the optical signals in a reliable and relatively inexpensive manner.